Reducing solution for the deposition of metallic films



Patented June 7, 1938 UNITED STATES REDUCING SOLUTION FOR THE DEPOSI- TION OF METALLIC FILMS Delaware J. Hood and Joseph H. Winterburg, Philadelphia, Pa., assignors to Phoebe Phillips Prime, Paoli, Pa.

No Drawing.

Application November 21, 1936,

Serial No. 112,084

Claims.

This invention relates to reducing solutions for the deposition of metallic films and particularly to an improved reducing solution for use in the deposition of silver films for mirrors and the like.

Heretofore metallic films such as silver have been deposited by the action of various reducing solutions on metallic compounds such as silver nitrate. The use of these prior art reducing solutions is often accompanied by a number of disadvantages. For instance, the deposited metallic film often exhibits so-called pin holes. Moreover the films so formed are not sufficiently durable to withstand quick handling or rough treatment. A further difliculty encountered in most of the metal deposition processes heretofore in use has been the production of troublesome scum formations. For instance in the deposition of silver films for mirrors and the like a sludge forms on the back of the film that has been deposited. This sludge is so adhesive as to make its removal a practical impossibility. It resists removal prior to the time when the film is dry and after drying it adheres in the form of a hard cake. A chamois makes but a slight impression on this cake and therefore it is left for the final protective coating to cover it. It is thought that the presence of this cake is responsible for subsequent defects in the metallic film. It is also probable that the presence of such large amounts of adhesive scum renders the deposited film less durable and less brilliant than would otherwise be the case, since small quantities of the scum distributed throughout the deposited metallic film would affect its cohesive properties and dull its brilliance somewhat.

Many attempts have been made to improve the quality of metallic films of the type referred to above, the aim being to produce a uniformly hard film which will at the same time be brilliant. Improvements have also been sought in the direction of increasing the coverage obtainable by the use of a given quantity of metallic compound. The present invention contemplates improving 45 the finished metallic film by furnishing an improved reducing solution together with a process for the deposition of improved metallic films by the use of this reducing solution.

Therefore, the principal object of the present invention is the furnishing of an improved process for the deposition of metallic films. A further object is the furnishing of a process for the production of an improved reducing solution. A still further object is to provide an improved reducing 55 solution, the use of which will obviate many of .tartrate, and 2 ounces of silver nitrate.

the disadvantages inherent in the prior art methods for the deposition of metallic film.

The improved reducing solution which is the subject of this invention may be prepared in the following manner. A solution is first made up 5 comprising one gallon of distilled water, 4 pounds of Rochelle salts, to wit: sodium potassium In this solution, the added silver nitrate is decidedly a minor, small proportion or percentage of the sodi- 10 um potassium tartrate in the solution. This solution is placed in a suitable container and an electric current applied by means of carbon electrodes or the like. We have found that a 110 volt, 60 cycle, alternating current is suitable for 15 this purpose. When the current has been flowing for a time, the solution will begin to turn black. manifesting the formation of insoluble matter resulting from the change(s) gradually brought about by the current passing through the solution under the moderate applied voltage. The progressive visible blackening of the liquid goes on until (after the passage of current for a sufficient time) the solution has reached a condition of uniform blackness, when the current is turned off and the carbon electrodes may then be removed. The solution so obtained is allowed to stand for a time sufllcient to bring about a thorough settling of the materials formed by the action of the electric current. This settling period may vary from a few days to three weeks. At the end of this time the sludge which has formed at the bottom of the container is removed by filtration and the filtrate is then diluted to the required specific gravity. This value varies for different conditions of use. For use in the formation of silver films for mirrors and the like according to the process outlined below, we have found that a specific gravity of 1.141 is most suitable. On 40 dilution to this figure the filtrate becomes the reducing solution referred to in the process for the deposition of metallic films as set forth below. Silver may be recovered by any of the well known methods from the sludge from which the filtrate has been separated.

In the deposition of metallic film by use of the reducing solution prepared as described above,

a mixture of three different solutions is used. These are referred to as solutions A, B and C 5 in the description which follows.

Solution A is made up by dissolving 4 ounces of silver nitrate in 3 ounces of ammonia of 26 B. To this silver-nitrate solution 15 ounces of distilled water is added and the solution is filtered. To the filtrate ounces of distilled water are added and the solution is thoroughly agitated.

Solution 13 is made up by adding 48 ounces of distilled water to 32 ounces of reducing solution made up according to this invention as described above.

Solution C is made up by mixing 1 ounces of tartaric acid with 50 ounces of distilled water and filtering.

One method for the production of a metallic film by the use of the three solutions described above is to mix thoroughly 5 ounces of solution A with 96 ounces of distilled water to which mixture 5 ounces of solution B is added and thoroughly mixed. To the mixture so obtained 2 ounces of solution C -is immediately added, and after agitation the resulting solution is poured on the surface to be treated, which is preferably maintained at a temperature of from to F. At temperatures within this range a metallic coating will form in from 15 to 20 minutes. A second coating may be applied after the surface of the first coating has been cleaned with a chamois. The second coating will be deposited in from 10 to 15 minutes.

A metallic film made according to this process and with the improved reducing solution of this invention has many advantages over the metallic films of the prior art. A film so formed has resisting qualities of its own in that it is tighter and firmer, and will withstand friction and rough handling to a greater degree than films deposited under prior mthods. The finished product may be handled more quickly and with less chance of injury by rough treatment than the prior art metaliic' films. Moreover. the metallic film is lighter, more durable, and more brilliant than other films and gives a truer reflection when used in the making of mirrors. It is also tough enough so that a chamois skin will not scratch it and at the same time it is free from so-called pin holes. Finally, in the use of this improved reducing solution maximum coverage for a given quantity of silver nitrate is obtained and at the same time there is a marked reduction in the amount of troublesome scum formations. This reduction in the amount of scum is very marked. Moreover, the slight amount of scum that does form is free fiowing and it can therefore be removed prior to the time when the metallic film has dried much easier than is the case with the scum formations of the prior art processes. Moreover, the thin cake which forms when the metallic film of the present invention dries out can be removed, for the most part, by a chamois, due to the fact that it is much less adhesive than the hard cake formed by prior art methods.

While the present invention has been described in detail with reference to a specific type of reducing solution and metallic film, it is to be understood that the same ben its are to be obtained in the deposition of metallic films of other kinds. Moreover, it is to be understood that various changes in the amounts of the ingredients as well as the type of electric current used are within the contemplation of the present invention and should not be deemed to constitute a departure from the spirit of the invention as hereinafter claimed.

Having thus described our invention, we claim:

1. A process of preparing an improvedreducing solution, useful in producing silver mirrors; which process comprises passing electric current through an aqueous solution of sodium potassium tartrate, and silver nitrate, under a moderate applied voltage which gradually brings about the formation of insoluble matter, for a sufficient length of time to produce a condition of uniform visible blackness in the liquid; and removing the insoluble matter thus formed from the liquid.

2. A process as set forth in claim 1 wherein, in removing the insoluble matter following the passage of electric current through the solution, the insoluble matter is allowed to settle and is thereafter filtered off from the liquid.

3. A process as set forth in claim 1, further characterized in that the silver nitrate in the solution treated is but a small percentage of the sodium potassium tartrate.

4. A process as set forth'in claim 1, further characterized in that the silver nitrate in the solution treated is but a small percentage of the sodium potassium tartrate, and in that the electric current passed through the solution is alternating current.

5. A reducing solution comprising an aqueous solution prepared according to the process of claim 1, which will reduce silver nitrate forming a tight, brilliant, durable metallic film on glass, and which is free from troublesome scum formation in use.

DELAWARE J. HOOD. JOSEPH H. W'INTERBUM. 

